rxperf: print achieved transfer rate

Prints the client transfer rate in appropriate units (kbit/s, Mbit/s, or Gbit/s).

Reviewed-on: http://gerrit.openafs.org/8758
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 07d83cd473146a0db9c2ed521b6fa6ba607b0d40)

Change-Id: Iacd6dedd035265062373ac700fe9537db3ce0478
Reviewed-on: http://gerrit.openafs.org/8933
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>